1. Field of the Invention
The present invention relates to an imaging optical system, an imaging optical device, and a digital apparatus. For example, the present invention relates to a compact but large-diameter standard lens of an inner focus type suitable for an interchangeable lens digital camera that captures a video image of an object with an imaging element (a solid-state imaging element such as a CCD (Charge Coupled Device) image sensor or a CMOS (Complementary Metal-Oxide Semiconductor) image sensor, for example), an imaging optical device that outputs a video image of an object captured with the standard lens and an imaging element as an electrical signal, and a digital apparatus having an image input function such as a digital camera equipped with the imaging optical device.
2. Description of the Related Art
The compactness of mirrorless interchangeable lens cameras that are single-lens reflex cameras minus flip-up mirrors has been welcomed by users recently, and the market of such mirrorless interchangeable lens cameras is now expanding. Some of the mirrorless interchangeable lens cameras are not capable of using phase detection AF (autofocus), unlike the majority of conventional single-lens reflex cameras. Such a camera performs focusing by scanning a focusing group and detecting the location where the highest contrast is achieved, or uses so-called contrast AF.
The problem in such a case is the weight of the focusing group. In the case of phase detection AF, information from the AF sensor is used, and the moving distance required for the focusing group to perform focusing can be calculated. Accordingly, the focusing group can be moved in accordance with the calculated moving distance. In the case of contrast AF, on the other hand, the information obtained from the AF sensor is only the contrast value at that moment, and performs a focusing operation by moving the focusing group and searching for the location where the highest contrast is achieved while reading variation in contract at given times. Therefore, the moving distance of the focusing group before focusing in the case of contrast AF is much longer than that in the case of phase detection AF.
In view of the above, a reduction in the weight of the focusing group is critical for an imaging optical system to cope with contrast AF (see JP 2013-3324 A and JP 2012-242472 A, for example).
JP 2013-3324 A discloses a structure that includes a first lens group of positive lenses, a second lens group of negative lenses, and a third lens group of positive lenses, and moves the second lens group toward the image side at a time of focusing. So as to reduce the weight of the focusing group, the second lens group is formed with one to three lenses. However, the second lens group has a high magnification, and the first lens group has a long total length so as to correct coma aberration that occurs in the second lens group. As a result, the total lens length becomes undesirably long.
JP 2012-242472 A also discloses a structure that includes a first lens group of positive lenses, a second lens group of negative lenses, and a third lens group of positive lenses, and moves the second lens group toward the image side at a time of focusing. So as to reduce the weight of the focusing group, the second lens group is formed with one negative lens. However, the stop is placed in the first lens group. As a result, the variation caused in coma aberration due to an off-axis light flux at a time of magnifying becomes larger.